The present invention relates to an improved automatic profile-copying device. More particularly, it relates to such a device specifically adapted for use in association with machines for milling flat pieces of different shapes with a mixed-line or irregular contour of the type using two opposed driven rollers for advancing a profile to which is secured the workpiece to be worked upon. The device according to the invention is particularly suited for machines of the type used to work on elements of wood and similar materials.
As is well known, grooving, profiling and shaping operations are carried out on machines with the tool shaft projecting from the working plane of the machine, known as milling machines with a vertical lower shaft, or else on machines with the tool shaft placed over the working plane of the machine, known as milling machines with a vertical upper shaft. The device according to the present invention is suitable for application on both typs of such milling machines, as well as on similar machines which requre the employment of an operating unit to perform an operation upon differently-shaped workpieces having mixed-line or irregular contours.
Moreover, as is also known with respect to devices of this type (which include two opposed rollers, of which at least one is driven and between which rollers the profile is advanced), in order to obtain a suitable advancing force at least the driven roller is covered with a high-friction enhancing material (for example, rubber or the like), while external means are provided to compress the profile between the rollers. The fundamental characteristic of the devices of this type lies in the fact that one roller, referred to as the "central roller", has its axis fixed and concentric with respect to the axis of the tool shaft, while the second roller, referred to as the "auxiliary or side roller", has its axis parallel and spaced from that of the central roller and is movable relative thereto, so as to guarantee, on one hand, easy insertion and removal of the profile between the rollers at the beginning and at the end of the operation and, on the other hand, the presence of a constant pressure on the profile, as is required for advancing the profile therebetween even if the profile thicknes varies. In fact, the means providing the force pressing the two rollers against the profile, always act elastically on the side roller, while the central roller is maintained in a fixed, accurate, coaxial position relative to the tool shaft of the milling machine to which the device is associated, so as to ensure an accurate copying of the intended milling configuration or contour.
The fundamental problem with this type of device is that, while the central roller rotates at a fixed speed so as to ensure a constant speed of advancement of the workpiece against the tool, the side roller does not rotate at a speed which should be variable according to the shape of the profile (except, of course, for the rectilinear profile portions or lengths). Up to now, this problem has been solved in various ways, none of which, however, has proved to be fully satisfactory. According to a first solution, only the central roller is driven and the side roller is freey rotatable. In this case, however, although there is the advantage of a constant advancing speed of the workpiece with respect to the tool, there is also the great disadvantage that it does not, for the most part, allow for the rotation of the profile in a manner correponding to its configuration and, in particular, any of its sharply bent points. In fact, the central roller acts with a torque for causing the rotation of the profile wich is only equal to the friction force onto its surface multiplied by its radius.
It has also been proposed to drive only the side roller, which acts to cause the rotation of the profile with a torque equal to the friction force on its surface multiplied by the radius of the central roller plus the thickness of the profile. It has then been possible to obtain a far higher capability of rotation than in the previous case. This, however has led to the drawback that the speed of advancement of the workpiece towards the tool is no longer constant (except for rectilinear profile lengths or portions). In particular, compared to the speed with the rectilinear profile lengths, there is a deacceleration with respect to convex profile lengths and an acceleration with respect to concave profile lengths.
In order to eliminate the drawbacks deriving from the driving of only one of the two rollers, it has been proposed to drive both rollers, at different speeds and, preferably, with the central roller driven at a constant speed and the side roller driven at a variable speed (i.e., by employing means which allow the side roller to rotate at a variable speed). With such a double roller drivn device, the profile can move according to an appropriate law or more appropriate travel path to improve the efficiency of the working operation. In addition, it is, of course, possible to obtain a higher drawing or pulling force, which is very useful in the case of relatively heavy workpieces and which is, in any event, always appropriate. A first way of realizing this concept is by introducing a differential mechanism allowing the shafts of the two rollers to rotate at different speeds driven by a single motor, the shaft of which rotates at constant speed. In this case, there is a very good drawing or pulling force but the speed of advancement of the piece towards the tool is not constant, since the path with constant advancing speed is not that in correspondence with the tool, but that coinciding with the center line of the profile thickness. A second, more practical method takes into account the opportuneness of rotating the central roller at a fixed speed and of preventing, at the same time, the profile from sliding with respect to such roller. For this purpose, a friction joint is inserted in the transmission of the side roller (which is rotated at the same time as the central roller, also in this case by a single motor). The friction joint allows the side roller to rotate at different speeds always providing a torque. Of course, to make sure that the friction joint does not act as brake instead of acting as drive mechanism motor, the speed of the inlet shaft of the joint itself is kept slightly above the maximum speed which may be required during movement of the profile, so that the friction joint continuously slides only in the desired direction. This system solves the problems of the aforementioned devices, but it has the rather considerable drawback of requiring more power than that typically required for feeding the profile, since a notable amount of power is lost in the friction joint. Moreover, this latter point constitutes a weak point of the device from the point of view of construction and maintenance.